Civil Defense SirenADDPMP266
A civil defense siren (also known as an air-raid siren or tornado siren) is a siren used to provide an emergency population warning to the general population of approaching danger. It is sometimes sounded again to indicate the danger has passed. Some sirens (especially within small towns) are also used to call the volunteer fire department when needed. Initially designed to warn city dwellers of air raids in World War II, they were later used to warn of nuclear attack and natural destructive weather patterns such as tornadoes. The generalized nature of sirens led to many of them being replaced with more specific warnings, such as the broadcast-based Emergency Alert System and the Cell Broadcast-based Wireless Emergency Alerts and EU-Alert mobile technologies.
A mechanical siren generates sound by spinning a slotted chopper wheel to interrupt a stream of air at a regular rate. Modern sirens can develop a sound level of up to 135 decibels at 100 feet (30 m). The Chrysler air raid siren, driven by a 331-cubic-inch (5.4-liter) Chrysler Hemi gasoline engine, generates 138 dB at 100 feet (30 m).
By use of varying tones or binary patterns of sound, different alert conditions can be signaled. Electronic sirens can transmit voice announcements in addition to alert tone signals. Siren systems may be electronically controlled and integrated into other warning systems.